1. Field of the Invention
The present invention relates to an optical mode splitter for effectively separating plural waveguide modes which propagate through an optical waveguide and a magneto-optical signal detecting device using such a mode splitter.
The present invention also relates to a magneto-optical information read/write device, more particularly it relates to an optical pick up unit of the magneto-optical information read/write device such as a magneto-optical disk drive, a magneto-optical-card drive and a magneto-optical tape drive.
2. Description of the Related Art
As a mode splitter in an optical waveguide, there have been developed a groove type mode splitter, a ninety-degree Bragg type mode splitter.
However, with regard to the groove type mode splitter, a problem is that the extinction ratio is sensitive to angular deviation. Also, with regard to the ninety-degree Bragg type mode splitter, a problem is that the extinction ratio is sensitive to the wavelength fluctuation. And with regard to the directional coupler type mode splitter, a problem is that it is required to control the coupling length very severely. Therefore, it is difficult to use the above mode splitters in an optical integrated device such as an optical pick up or optical communication element.
The inventors of this application have proposed a mode splitter which enables to separate plural waveguide modes propagating in an optical waveguide efficiently by using a tapered junction in Japanese Patent Application Laying Open (KOKAI) Nos. 64-4706 and 64-4707.
The proposed mode splitter has a very simple structure and by which it is possible to separate the waveguide modes efficiently. Furthermore, in principle, the mode splitter can separate any modes at any incident angle.
However, actually, the range of the incident angle, in which the modes can be separated, is restricted by the refractive index and thickness of the waveguide. To separate the modes efficiently irrespective of fabrication errors of the refractive index and thickness of the waveguide and a misalignment of the optical elements in the waveguide, it is required to widen the above incident angle range.
On the other hand, a magneto-optical disk device has been briskly developed as a rewritable high density memory device. The magneto-optical disk system enable to read information by detecting a rotation angle of a polarization of a reflected light from the magneto-optical recording medium due to the Kerr effect.
Since the rotation angle of the polarization due to the Kerr effect is very small, it is necessary to use magneto-optical signal detection unit with a high accuracy photodetector or optical differential detecting system to obtain high S/N (signal/noise) ratio. Conventional magneto-optical signal detection unit comprises bulk type optical elements such as a polariser, a prism and a lens, which makes it difficult to arrange them at a precise position relative to each other and form a compact structure.
To solve the problems of the bulk optical system, Nishihara laboratory of Osaka University has proposed "A waveguide type differential detection device for magneto-optical disk pick up" in which a magneto-optical signal detection system is integrated on a thin film waveguide.
However, in the above waveguide type detection device, the S/N signal is poor because of an asymmetric intensity distribution of the reflected light from the magneto-optical disk at the grating coupler.
3. Summary of This Invention
The patent invention was made considering the above-mentioned points of the related art.
It is therefore an object of the present invention to provide a mode splitter having a wide range of mode separatable incident angle of light and enabling efficiently separate plural waveguide modes irrespective of the fabrication errors of the refractive index and thickness of the waveguide and the misalignment of the optical elements in the waveguide.
Another object of the present invention is to propose a magneto-optical signal detection device integrated on a thin film waveguide by which the excellent S/N signal is obtained.
FIGS. 21a and 21b explain the state where light rays propagate within an optical waveguide which comprises a substrate layer having a refractive index of n.sub.s, an optical waveguide layer having a refractive index of n.sub.f and a clad layer having a refractive index of n.sub.c, wherein n.sub.f &gt;n.sub.s, n.sub.c. Z in FIG. 21b represents the propagation direction of the light rays in FIG. 21a.
Assuming that a critical angle on a first boundary face between the clad layer and the optical waveguide layer is .theta..sub.c and a critical angle on a second boundary face between the substrate layer and the optical waveguide layer is .theta..sub.s in a case where light rays enter an interior of the optical waveguide layer at an angle of .theta. with respect to the first boundary face as shown in FIG. 21a, .theta..sub.c and .theta..sub.s are respectively given by the following formulae: EQU .theta..sub.c =sin.sup.-1 (n.sub.c /n.sub.f) and EQU .theta..sub.s =sin.sup.-1 (n.sub.s /n.sub.f).
If the incident angle .theta. is greater than both of .theta..sub.s and .theta..sub.c, the light rays propagate while repeating total reflections of the first and second boundary faces. Such a propagation is termed a guided mode.
The phenomenon of a guided wave represented by the total reflection of light rays can be considered in connection with wave which is another property of light.
Next, consider the case in which light rays propagate at an angle of .theta. with respect to the boundary within the optical waveguide layer as shown in FIG. 21a. The light rays propagating in such a state are called a plane wave from a viewpoint of wave.
If the wavelength of the plane wave in a vacuum is taken to be .lambda., then k.sub.o =2.pi./.lambda. is referred to as a propagation constant in the vacuum. Components of the propagation constant in X and Y directions as shown in FIG. 21b are respectively represented by the following formulae: EQU k.sub.X =k.sub.o n.sub.f cos .theta. and EQU k.sub.Y =k.sub.o n.sub.f sin .theta..
Especially, a component .beta. of the propagation constant in Z direction where the light rays propagate is represented by the following formula: EQU .beta.=k.sub.o n.sub.f sin .theta.,
and the constant .beta. is generally termed the propagation constant of guided modes.
Among these guided modes, a guided mode having a component of an electric field only in the Y direction is termed a TE mode, and a guided mode having a component of a magnetic field only in the Y direction is termed a TM mode.
The above-mentioned mode splitter can separate plural modes propagating through the waveguide having a region A and a region B, in which a mode i having propagation constants .beta..sub.Ai for the region A and .beta..sub.Bi for the region B and another mode j having propagation constants .beta..sub.Aj for the region A and .beta..sub.Bj for the region B are propagation.
The propagation constants satisfy the following formulae: EQU .beta..sub.Ai &gt;.beta..sub.Bi ( 1) EQU .beta..sub.Aj &gt;.beta..sub.Bj ( 2) EQU (.beta..sub.Bi /.beta..sub.Ai)&gt;(.beta..sub.Bj /.beta..sub.Aj)(3)
The mode splitter has a first taper boundary portion for conjunction from the region B to the region A and a second taper boundary portion for conjunction from the region A to the region B, each taper boundary being gradually thinned toward an outer end thereof.
An angle .theta. between the first and second taper boundary being larger than a critical angle for mode i, i.e., EQU .theta.&gt;sin.sup.-1 (.beta..sub.Bi /.beta..sub.Aj)
An incident angle a of the propagating light including the mode i and j at the first taper boundary satisfying the following formula: EQU Sin.sup.-1 {(.beta..sub.Ai /.beta..sub.Bi) sin (.theta.-.theta..sub.ci)}&lt;.alpha.&lt;sin.sup.-1 {(.beta..sub.Aj /.beta..sub.Bj) sin (.theta.-.theta..sub.cj)} (4)
wherein
.theta..sub.ci =sin.sup.-1 (.beta..sub.Bi /.beta..sub.Ai);
which is a critical angle of the mode i; and EQU .theta..sub.cj =sin.sup.-1 (.beta..sub.Bj /.beta..sub.Aj);
which is a critical angle of the mode j.
In accordance with the mode splitter of the present invention mentioned above, plural modes are separated first due to the difference of the refraction angle at the time of refraction in the first taper portion, and after that, the modes are separated by the difference of the critical angle of each mode in the second taper portion. Therefore, it becomes possible to widen the mode separatable incident angle range and separate the modes efficiently irrespective of the fabrication errors of the refractive index and thickness of the waveguide and the misalignment of the optical elements in the waveguide compared with the related art structure wherein the modes are separated with the use of only one taper boundary portion.
The above magneto-optical signal detecting device comprises:
an optical waveguide coupling portion to couple two polarization components of reflected light from a magneto-optical recording medium, which components are perpendicular to each other, into a waveguide, and
a TE-TM mode separating portion to separate a TE mode and a TM mode efficiently, which is an application of the above-mentioned mode splitter of the present invention. The TE-TM mode separating portion is also realized by the related art of mode splitter in Japanese Patent Application Laying Open (KOKAI) Nos. 64-4706 and 64-4707.
The principle of the above-mentioned optical waveguide coupling is briefly described below.
There are a grating coupler method and a prism coupler method for coupling a reflected light from a magneto-optical recording medium into an optical waveguide. In accordance with these methods, the reflected light is coupled by the phase matching of the incident wave and the waveguide mode. Therefore, any required waveguide mode can be coupled by adjusting the incident angle to the synchronous angle corresponding to the effective index of the waveguide mode.
While, each waveguide mode has a respective different effective index and it is possible to put the effective index of a TE mode close to that of a TM mode by thickening the waveguide. In the case that the effective indices of a certain TE mode (TEi mode) and a certain TM mode (TMj mode) are almost equal, both of the TEi mode and the TMj mode can be coupled by the grating coupler or the prism coupler at the same incident angle which is the synchronous angle corresponding to the effective index of the TEi mode (or TMj mode).
Therefore, it becomes possible to couple two polarization components of the reflected light from the magneto-optical recording medium, which are perpendicular to each other, into a waveguide as the TE mode and the TM mode simultaneously.
Therefore, by constituting an optical system comprising such an optical waveguide coupling portion and the TE-TM mode separation portion mentioned above in one waveguide, it becomes possible to couple two polarization components of the reflected light from the magneto-optical recording medium, which are perpendicular to each other, into a waveguide as the TE mode and the TM mode simultaneously, and after that separate the TE mode and the TM mode in the TE-TM mode separation portion.
Thereby, it becomes possible to obtain a good signal which is free from the noise caused by an asymmetric intensity distribution of the reflected light from the magneto-optical disk.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.